Ferromagnetic Resonance Studies of MnZn Ferrites/Polymer Composite Materials.

We characterize MnZn ferrite particles embedded in a polymer for use in low frequency EMF emissions shielding. The ferrites particles are approximately 1.2 µm in diameter and embedded in PVC resin in various concentrations: 10% - 70% by weight. The composite undergoes an extrusion process which creates a 0.6 mm thick slab and orders some of the particles along the extrusion direction. This creates an easy axis along the extrusion direction with an associated anisotropy. We characterized the ferromagnetic resonance absorption peaks with broad-band FMR, 1-30 GHz, and cavity based FMR systems. Comparing the results to the expected FMR peaks for measurements along the easy and hard axes, and normal to the slab, using the Landau-Lifshitz-Gilbert equation provides some interesting irregularities. Samples with high ferrite concentrations, Kittel's equation for thin film resonance can be used to describe the FMR frequency vs. field dependence. For low ferrite concentrations the resonance conditions have to be modified to account for an effective thickness beyond the normal filling factor correction associated with presence of a matrix. These results indicate the effective demagnetizing factors, determined by the spatial extend of the RF fields, can describe the observed FMR absorption.